Anecdotal evidence

Anecdotal evidence is evidence based only on personal observation, collected in a casual or non-systematic manner.
When used in advertising or promotion of a product, service, or idea, anecdotal reports are often called a testimonial, which are highly regulated in some jurisdictions.
When compared to other types of evidence, anecdotal evidence is generally regarded as limited in value due to a number of potential weaknesses, but may be considered within the scope of scientific method as some anecdotal evidence can be both empirical and verifiable, e.g. in the use of case studies in medicine. Other anecdotal evidence, however, does not qualify as scientific evidence, because its nature prevents it from being investigated by the scientific method. Where only one or a few anecdotes are presented, there is a larger chance that they may be unreliable due to cherry-picked or otherwise non-representative samples of typical cases. Similarly, psychologists have found that due to cognitive bias people are more likely to remember notable or unusual examples rather than typical examples. Thus, even when accurate, anecdotal evidence is not necessarily representative of a typical experience. Accurate determination of whether an anecdote is typical requires statistical evidence. Misuse of anecdotal evidence in the form of argument from anecdote is an informal fallacy and is sometimes referred to as the "person who" fallacy ("I know a person who..."; "I know of a case where..." etc.) which places undue weight on experiences of close peers which may not be typical.
Scientific context
In science, definitions of anecdotal evidence include:
"casual observations or indications rather than rigorous or scientific analysis"
"information passed along by word-of-mouth but not documented scientifically"
"evidence that comes from an individual experience. This may be the experience of a person with an illness or the experience of a practitioner based on one or more patients outside a formal research study."
"the report of an experience by one or more persons that is not objectively documented or an experience or outcome that occurred outside of a controlled environment"
Anecdotal evidence can have varying degrees of formality. For instance, in medicine, published anecdotal evidence by a trained observer (a doctor) is called a case report, and is subjected to formal peer review. Although such evidence is not seen as conclusive, researchers may sometimes regard it as an invitation to more rigorous scientific study of the phenomenon in question. For instance, one study found that 35 of 47 anecdotal reports of drug side-effects were later sustained as "clearly correct."
Anecdotal evidence is considered the least certain type of scientific information. Researchers may use anecdotal evidence for suggesting new hypotheses, but never as validating evidence.
If an anecdote illustrates a desired conclusion rather than a logical conclusion, it is considered a faulty or hasty generalization.
In any case where some factor affects the probability of an outcome, rather than uniquely determining it, selected individual cases to prove nothing; e.g. "my grandfather smoked two packs a day until he died at 90" and "my sister never smoked but died of lung cancer". Anecdotes often refer to the exception, rather than the rule: "Anecdotes are useless precisely because they may point to idiosyncratic responses."
In medicine, anecdotal evidence is also subject to placebo effects.
See also
References
Informal fallacies
Philosophy of science
Skepticism
Evidence
Testimony
Inductive fallacies
Pseudoscience
Diversionary tactics
Misuse of statistics